Fusion Welding of Aluminum and Its Alloys: Part Two

Sažetak:

The use of some form of back-up to maintain and shape in position molten aluminum on full fusion butt joints welded from one side only are numerous and take many forms.
Because of the high degree of expansion during welding, especially where it is impractical to use jigging, tack welding becomes necessary to maintain components in position. These tack welds must be made in the manner in which the main weld is to be deposited.

Weld Backing Aids

The use of some form of back-up to maintain and shape in position molten aluminum on full fusion butt joints welded from one side only are numerous and take many forms.

Temporary Backing Bars. These are made of mild steel, stainless steel, or copper, and are placed under the joint and subsequently removed when the weld has been completed. Care must be taken when using bars made of mild steel or copper, because it is possible to contaminate the weld.

Burnt spots caused by arc impingement on to these bars produce iron and aluminum/copper pick-up respectively. These areas will be brittle and sensitive to corrosion and must be avoided. When using stainless steel bars, however, these problems do not arise and are, therefore, recommended whenever possible. It is usual for the bar to have a suitable dimensional radiused groove, machined along its length to mould the penetration bead to a desired geometrical shape.

To ensure that an adequate penetration bead can be formed, the groove must not be too shallow or the possibility of oxide entrapment and other unwanted foreign matter will inherently accumulate at the root of the joint and result in porosity and lack of fusion. Conversely, a groove that is too deep can cause excessive penetration bead formation and slow up weld travel speed which can lead to lack of root fusion in the form of cold lapping.

Preheating of the backing bar is a practice recommended, more so when thicker plates are welded, and has proved to be a most satisfactory method of controlling the chilling effects. The advantages of this action is twofold. First, moisture is dried out of the backing bar which prevents the possibility of hydrogen entrapment causing porosity and, secondly, rapid chilling is eliminated.

Cold weld starts are also cancelled out and the penetration bead will be found to assume a smooth and regular shape. Preheating temperatures need only to be enough to dry out any residual moisture, say, 100°C, which can be accomplished by heating torches. It is worth remembering that more than one bar may be needed at different welding stations during production, and the use of a specially designed heating oven or heating coils are worth considering. The bars can be left for an unstipulated length of time at a constant temperature until required.

Permanent Backing Strips. This type of backing can take two forms. The first type consists of a separate strip of material similar to that being welded which is left in place after welding to become part of the joint. The second form is where the backing strip is an integral part of the joint preparation, and examples of these two backing aids. In each instance, good fit-up between the backing members and the plates to be welded is essential, otherwise weld defects such as lack of fusion, suck back, and oxide inclusions will prevail.

Gas Backing. An inert gas, i.e., argon or helium can be used as a complementary means of backing and can be passed to the underside of the joint via a hollow backing bar, or, with pipe joints, purging the internal bores. Extreme care has to be taken with this method, because excessive gas flow rates can cause a back pressure build up which can promote suck back of the penetration bead.

Back Chipping

With joints requiring full penetration, e.g., those with single or double V and U preparations which need to be welded from each side, back chipping of the root, once the first side has been completed, is often necessary. This can be accomplished by the use of milling and routing techniques, but pneumatic chisels are more commonly used.

The advantages of back chipping are invaluable, as unfused or partially fused metal at the root of the joint can be removed to a depth where sound metal exists. When carrying out back chipping operations, care must be taken to ensure that metal is removed cleanly by using the correct chipping tool. The use of diamond-nosed chisels, for example, is not suitable because of the undesirable V groove they produce.

Tack Welding

Because of the high degree of expansion during welding, especially where it is impractical to use jigging, tack welding becomes necessary to maintain components in position. These tack welds must be made in the manner in which the main weld is to be deposited.

In some instances, these tacks will have to be removed as welding progresses so as not to interfere with the initial weld run. When tack welds are not removed, precautions must be taken, such as tapering them to facilitate smooth remelting as welding progresses, thus preventing the possibility of lack of fusion areas occurring.

Control of Distortion

The problem of distortion is as inherent with aluminum as it is with other common metals when they undergo welding. While it is impossible to prevent distortion occurring on a weldment, there are ways of controlling it by adopting a carefully planned procedure.

It is not possible to specify in detail such a procedure, but the following factors make significant contributions towards controlling distortion on aluminum fabrications:

Always try to allow individual plates and sections some degree of freedom of movement and weld from a fixed to a free end to prevent possible build up of locked-in stresses.

Plan all welding sequences so that they are balanced symmetrically about the neutral axis of a joint.

Avoid wide included angle preparations and excessive root gaps on butt joints, keeping the excess weld metal to a minimum. Weld sizes on fillet joints should be kept to a controllable size, i.e., as small as the design requirements will allow.

Once a weld run has been commenced it should be completed without unnecessary interruptions.

Use the highest welding speeds possible compatible with the material thickness and welding position consistent with proper fusion of the joints; thus keeping the heat input from spreading throughout the components. To achieve this, the MIG process is to be preferred to AC TIG; DC TIG can be acceptable when mechanized.

Finally, the effective use of chills, jigs, and fixtures is recommended wherever possible.